1. Field of the Invention
The present invention relates to an image forming device and method which form images corresponding to image data on recording sheets in accordance with an electrophotographic process. In particular, the present invention relates to an image forming device and method at which continuous double-sided printing is possible.
2. Description of the Related Art
Image forming devices using an electrophotographic process are generally used in image formation at printers, copiers, or fax machines which form images on recording sheets, or multi-function devices which combine the functions of the aforementioned devices, or the like. In an image forming device, a toner image is formed by developing, using toner, an electrostatic latent image formed on a photosensitive drum. Thereafter, in the image forming device, the toner image is transferred to a recording sheet, and thereafter, the toner image is fixed to the recording sheet by being heated while pressure is applied to the toner image together with the recording sheet by using fixing rollers.
Such an image forming device is provided with developing units corresponding of the respective colors of C (cyan), M (magenta), Y (yellow), and K (black). At each of the developing units, a photosensitive drum is provided. The photosensitive drum is exposed such that a latent image is formed thereon, and thereafter, toner images are formed by carrying out toner development using toners of the respective colors.
Among image forming devices, there are those which are provided with an intermediate transfer body such as an intermediate transfer belt or the like. The toner images of the respective colors which are formed on the photosensitive drums are transferred in a superposed manner onto the intermediate transfer body such that a color toner image is formed. Thereafter, by transferring the toner image which is on the intermediate transfer body onto a recording sheet, a color image using toners of the respective colors of C, M, Y, K is formed on the recording sheet.
Such an image forming device can carry out so-called double-sided printing in which images are formed on the both surfaces (Side 1 and Side 2) of a single recording sheet. An image forming device which can carry out double sided printing is provided with a reversing section which switches the conveying direction of a recording sheet at which the image of Side 1 has been formed, and a circulating conveying path which returns the recording sheet, at which the image of Side 1 has been formed, to the sheet feeding side of an image forming section. Image formation on Side 2 is thereby possible.
There are image forming devices which, in order to be able to form images on a large number of recording sheets in a short period of time, continuously convey the recording sheets at predetermined pitches, and form images on the intermediate transfer body in accordance with the conveying pitches of the recording sheets.
In such an image forming device, when double-sided printing is carried out continuously onto a large number of recording sheets, for example, the images of Side 1 are formed onto a predetermined number of the recording sheets respectively, and when the recording sheets on which these images of Side 1 have been formed are returned to the sheet feeding side, they are placed between recording sheets on which images of Side 1 are to newly be formed, such that image formation at Side 1 and image formation at Side 2 are carried out in parallel. The produceability is thereby improved.
At this time, the order of forming the toner images is set in advance in accordance with the order of the sheets being fed to the image forming section (i.e., scheduling is carried out), such that the formation of the toner images is carried out in the set order.
Namely, at the time of carrying-out single-sided printing, as shown in FIG. 5A, scheduling is carried out so as to place, in order, an image P1 to be formed to a first recording sheet, an image P2 to be formed on a second recording sheet, an image P3 to be formed on a third recording sheet, . . . . In contrast, when double-sided printing is carried out, as shown in FIG. 5B, scheduling is carried out with a “pitch skip” being provided.
For example, FIG. 5B illustrates a case in which image formation of Side 2 is carried out, at a timing corresponding to six pitch skips, after image formation of Side 1. Namely, in this case, regions corresponding to six images (six sides of recording sheets) are provided between image P1a formed on Side 1 of the first recording sheet and image P1b formed on Side 2 of the first recording sheet. In this case, a region corresponding to one image is skipped (i.e., one pitch skip is provided) between image P1a formed on Side 1 of the first recording sheet and image P2a formed on Side 1 of the second recording sheet. Further, image P1b formed on Side 2 of the first recording sheet is placed between image P4a formed on Side 1 of the fourth recording sheet and image P5a formed on Side 1 of the fifth recording sheet.
Double-sided printing can be carried out efficiently by scheduling in this way.
On the other hand, there are image forming devices which switch on and off motors (DeveMotors) used in rotation of photosensitive drums, when a single-color image using K or the like for example and a full color image using the four colors of C, M, Y, K are formed.
When on/off switching of a DeveMotor or the like arises when forming single-color images and full-color images in this way, the recording sheets are apt to become foggy or dirtied by toner. In order to prevent this, scheduling is carried out so as to insert pitch skip(s) and change the intervals at which the images are placed. Further, in cases in which one pitch skip (a region corresponding to one image) is insufficient (FIG. 5C), scheduling must be carried out so as to insert two pitch skips (regions corresponding to two images).
Namely, as shown in FIG. 5D, if the images P1, P2 formed on the first and second recording sheets are single-colored K images and the image P3 formed on the third recording sheet is a full-color (four-color) image (or in the opposite case), scheduling is carried out so as to skip regions corresponding to two images (insert two pitch skips) between the image P2 and the image P3.
However, as shown in FIG. 5B, when double-sided printing is carried out, the position of the image of Side 2 is already determined when the image of Side 1 is formed. Thus, when, for example, two pitch skips must be provided in order to switch between a single-color image and a color image, if the image P1b to be formed on Side 2 of the first recording sheet is a single-color image and an image P2b of Side 2 of the second recording sheet is a color image, a predetermined pitch skip cannot be provided between the images P1b, P2b. 